Per FIG. 1, an opposed-piston engine includes at least one cylinder 10 with a bore 12 and longitudinally-displaced intake and exhaust ports 14 and 16 machined or formed therein. One or more fuel injectors 17 are secured in injector ports (ports where injectors are positioned) that open through the side surface of the cylinder. Two pistons 20, 22 according to the prior art are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is denominated as the “intake” piston because of its proximity to the intake port 14. Similarly, the piston 22 is denominated as the “exhaust” piston because of its proximity to the exhaust port 16.
Operation of an opposed-piston engine with one or more ported cylinders (cylinders with one or more of intake and exhaust ports formed therein) such as the cylinder 10 is well understood. In this regard, in response to combustion the opposed pistons move away from respective top dead center (TDC) positions where they are at their innermost positions in the cylinder 10. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions where they are at their outermost positions in the cylinder. The pistons may move in phase so that the intake and exhaust ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
In many opposed piston constructions, a phase offset is introduced into the piston movements. For example, the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BDC positions in a sequence in which the exhaust port 16 opens as the exhaust piston 22 moves through BDC while the intake port 14 is still closed so that combustion gasses start to flow out of the exhaust port 16. Then as the pistons continue moving away from each other, the intake piston 20 moves through BDC, causing the intake port 14 to open while the exhaust port 16 is still open. A charge of pressurized air is forced into the cylinder 10 through the open intake port 14, driving exhaust gasses out of the cylinder through the exhaust port 16. As seen in FIG. 1, after further movement of the pistons, the exhaust port 16 closes before the intake port 14 while the intake piston 20 continues to move away from BDC. Typically, the charge of fresh air is swirled as it passes through ramped openings of the intake port 14. With reference to FIG. 1, the swirling motion (or simply, “swirl”) 30 is a generally helical movement of charge air that circulates around the cylinder's longitudinal axis and moves longitudinally through the bore of the cylinder 10. Per FIG. 2, as the pistons 20, 22 continue moving toward TDC, the intake port 14 is closed and the swirling charge air remaining in the cylinder is compressed between the end surfaces 20e and 22e. As the pistons near their respective TDC locations in the cylinder bore, fuel 40 is injected into the compressed charge air 30, between the end surfaces 20e, 22e of the pistons. As injection continues, the swirling mixture of air and fuel is increasingly compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e as the pistons 20 and 22 move through their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations.
As illustrated in FIG. 2, fuel is directly injected through the side of the cylinder (“direct side injection”) into the cylinder bore and the movement of the fuel interacts with the residual swirling motion of the charge air in the bore.
In some aspects of two-stroke, opposed-piston construction, the nature of the operating cycle results in the uninterrupted application of compressive loads on bearings in the piston-to-crankshaft connecting linkages. These non-reversing loads prevent the separation of bearing surfaces, which blocks the flow of lubricant therebetween and limits the durability of such bearings. For example, opposed-piston engines with single-crankshaft constructions include linkages with pivoted rocker arms. In these constructions, a rocker arm pivots by means of a hinge-type plain bearing that fails quickly under non-reversing high compressive loads generated by combustion.
In some aspects of opposed-piston engines, a variable compression ratio (VCR) system is provided to increase fuel efficiency by dynamically changing the compression ratio in order to optimize it under varying load conditions. A high compression ratio generally improves engine operating efficiency but is limited by the high structural loads that result at high power conditions. A VCR system allows the advantages of high compression ratio where appropriate while allowing low compression ratio where needed. VCR is implemented by changing the combustion chamber volume at or near TDC of the pistons. In one example described in U.S. Pat. No. 2,357,031, an opposed-piston engine with single-crankshaft construction that includes linkages with pivoted rocker-arms, a VCR system changes the pivot points of rocker arms by eccentric elements that are fixed on rotatable pivot shafts. When a pivot shaft is rotated to change the pivot point, each piston rod and the piston attached to the piston rod are moved, which moves the TDC and BDC points of the piston. When the TDC point is changed, the combustion chamber space between the piston and its opposing mate changes accordingly. However, the VCR mechanism described in this patent utilizes a complex, manually-actuated construction that does not respond automatically to changing engine conditions.